Demetrius Zaarin is a human Grand Admiral from Coruscant, created for the 1994 LucasArts flight simulator Star Wars: TIE Fighter. He is a Star Wars Legends character, and he is the reason a great many people know there were twelve Grand Admirals at all.
Research
Zaarin headed Imperial starfighter research and development. The TIE Advanced and the TIE Defender — the latter still the most beloved Imperial fighter design outside the films — came out of his facility. The game's structure makes this personal to the player: you fly his prototypes for most of the campaign before you are sent to kill him.
The coup
In 3 ABY, after the death of Prince Xizor, Zaarin moved against Palpatine. His forces captured the Emperor over Imperial Center, and the coup was broken by reinforcements under Darth Vader and Vice Admiral Thrawn. Zaarin fled and spent the rest of his life running an insurgency against the Empire he had helped arm, raiding Imperial facilities for technology.
The cloaking device
His end came through the Vorknkx project. Thrawn allowed him to seize a cloaking-device-equipped vessel that had been fitted with a tracking beacon and sabotaged; when Zaarin engaged the cloak together with the hyperdrive, the ship destroyed itself in the Unknown Regions in 3.5 ABY. Timothy Zahn later name-checked him in Choices of One (2011), which is the only novel to mention him.